This application claims the benefit of Korean Patent Application No. 1999-16743, filed on May 11, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to an optical detecting sensor and, more particularly, to a thin film transistor (TFT) type optical detecting sensor.
2. Discussion of the Related Art
Generally, optical detecting sensors are used in facsimile, digital copying machines and fingerprint recognition systems as an image reader. In recent years, a TFT type optical detecting sensor has been suggested. The TFT changes its electrical characteristics depending upon the presence of light incident to the device. The TFT type optical detecting sensor is a system using such a characteristic.
FIG. 1 shows a plan view of a pixel of a conventional TFT type optical detecting sensor.
As shown in FIG. 1, an optical detecting sensor 100 comprises a window 8, through which light generated from a light source (not shown) passes, and a sensor TFT 6 for generating optical current by detecting the light which is transmitted through the window 8 and then reflected from an object (not shown) disposed on the optical detecting sensor 100.
Optical current generated from the sensor TFT 6 is stored in a storage capacitor 4 as charges, and the charges stored in the storage capacitor 4 are transmitted by a switching TFT 2 to an external driving circuit(not shown). In addition, a light shielding layer 9 for blocking light is formed over the semiconductor layer where the switching TFT 2 is located.
In addition, the storage capacitor 4 has a predetermined capacity to store light current generated in proportion to the intensity of light reflected from the object. Therefore, a predetermined area is required for the storage capacitor 4. The storage capacitor 4 comprises first and second capacitor electrodes 30 and 34 and a capacitor wire 30xe2x80x2 connecting the first capacitor electrode 30 to the corresponding capacitor electrode of the adjacent pixel.
The above optical detecting sensor will be described more in detail hereinafter with reference to FIG. 2.
A first metal layer is deposited on a substrate 1, then patterned to form a switching gate electrode 20, a first capacitor electrode 30 and a sensor gate electrode 40 on regions corresponding to the switching TFT 2, the storage capacitor 4 and the sensor TFT 6, respectively.
A first insulating layer 32 is deposited on the substrate 1 while covering the switching gate electrode 20, the first capacitor electrode 30, and the sensor gate electrode 40.
An intrinsic amorphous silicon layer and a doped amorphous silicon layer are consecutively deposited on the first insulating layer 32, then patterned to form semiconductor layers 26 and 46 of the switching TFT 2 and the sensor TFT 6, respectively.
A second metal layer is then deposited to form switching source and switching drain electrodes 22 and 24 on the region of the switching TFT 2, a second capacitor electrode 34 on a region of the storage capacitor 4, and sensor source and sensor drain electrodes 42 and 44 on a region of the sensor TFT.
A second insulating layer 16, and a protecting layer 10 are formed to protect the switching TFT 2, the storage capacitor 4 and the sensor TFT 6 from external moisture or impact. A light shielding layer 18 is formed between the second insulating layer 16 and the protecting layer 10 to protect the switching TPT 2 from being activated or degraded by external light.
Since the sensor TFT 6 operates by optical current in an off-state, negative voltage is always applied to the sensor gate electrode 40 to maintain the off-state.
In the above described optical sensor, the window 8 is an essential part in order to transmit light from the light source to an object and an amount of light passing through the window 8 is one of the most important parameters affecting light efficiency. In addition, the higher the signal vs. noise ratio (S/N ratio), the clearer the image becomes.
However, in the above described optical sensor, since the capacitor wire 30xe2x80x2, which is connected to the first capacitor electrode 30 in order to apply a voltage to the storage capacitor 4, covers a portion of light passing area, the area of the window is reduced, decreasing the amount of the light incident to the sensor TFT 6.
Accordingly, the present invention is directed to thin film transistor type photo sensor and method for fabricating the same that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a TFT type optical detecting sensor in which areas of a storage capacitor and a window are maximized, thereby improving a signal vs. noise ratio (S/N).
Another object of the present invention is to provide an optical detecting sensor which has a higher switching speed.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and the claims, as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, in a preferred embodiment. The invention provides a TFT type optical detecting sensor having a plurality of pixels, each pixel comprising a sensor drain wire and a switching source wire, each disposed on opposite sides of the pixel, a storage capacitor for storing charges, the storage capacitor comprising a first capacitor electrode and a second capacitor electrode, a switching TFT for directing the charges stored in the storage capacitor to an external system, the switching TFT comprising a switching gate electrode and switching source and drain electrodes each disposed along a side of the pixel in a substantially perpendicular direction to the direction along which the sensor drain wire is disposed, an area between the switching source and drain electrodes defining a switching TFT channel, a sensor TFT for generating optical current and supplying the optical current to the storage capacitor, the sensor TFT comprising a sensor gate electrode and sensor drain and source electrodes, and wherein the width of the channel of the switching TFT is substantially the same as by the length of a side of the pixel, excluding the widths of the switching source wire and sensor drain wire.
Also, the first capacitor electrode is electrically connected to the sensor gate electrode.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.